Method for producing tubular metal can body

ABSTRACT

A method of producing a tubular metal can body wherein the dissolving of metal in the contents of the can is prevented, said can body comprising a metal blank whose longitudinal marginal portions are joined to each other by a lap side seam through adhesive layers of a thermoplastic organic polymer, said both marginal portions of the metal blank each including two opposing surfaces, a cut end surface and shearing burr projecting from one corner of the cut end portion, said lap side seam being formed by the integral joining of an in-side adhesive layer in a continuous ribbon form of a thickness of from 15 to 200 microns which completely covers both the inner and outer surfaces of the marginal portion of the metal blank located inside the can body, cut end surface and shearing burr and adheres firmly to these surfaces, and an outside adhesive layer in a continuous ribbon form of a thickness of 5 to 100 microns which covers at least the inner surface of the marginal portion of the metal blank located outside the can body and adheres firmly to the surface said outside adhesive layer adhered firmly to the inner surface of the metal blank having a width larger than that of the lap side seam, and extending in the circumferential direction beyond the turning point of the inside adhesive layer at the seam portion and thus being firmly adhered to the inside of the metal blank.

"i tates atent [191 ()kubo et a1.

[ METHOD FOR PRODUCING TUBULAR METAL CAN BODY [22] Filed: Aug. 21, 1973 [21] Appl. No.: 390,255

Related US. Application Data [62] Division of Ser. No. 153,290, June 15, 1971, Pat. No.

[52] U.S. C1. 113/120 F, 220/75, 156/200 [51] Int. Cl B2ld 51/28 [58] Field ofSearch 113/120 R, 120 F, 120 K,

[56] References Cited UNITED STATES PATENTS 3,125,056 3/1964 Kaiser 113/120 F 3,213,890 10/1965 Battersby et a1... 138/170 3,239,402 3/1966 Ecklund et a1 1. 156/200 3,411,542 11/1968 Walsh et a1. 113/120 K 3,437,063 4/1969 Battersby 220/75 3,524,781 8/1970 Winterroth et al.,.. 113/120 F [451 Apr. 30, 1974 Wight [57] ABSTRACT A method of producing a tubular metal can body wherein the dissolving of metal in the contents of the can is prevented, said can body comprising a metal blank whose longitudinal marginal portions are joined to each other by a lap side seam through adhesive layers of a thermoplastic organic polymer, said both marginal portions of the metal blank each including two opposing surfaces, a cut end surface and shearing burr projecting from one comer of the cut end portion, said lap side seam being formed by the integral joining of an in-side adhesive layer in a continuous ribbon form of a thickness of from 15 to 200 microns which completely covers both the inner and outer surfaces of the marginal portion of the metal blank located inside the can body, cut end surface and shearing burr and adheres firmly to these surfaces, and an outside adhesive layer in a continuous ribbon form of a thickness of 5 to 100 microns which covers at least the inner surface of the marginal portion of the metal blank located outside the can body and adheres firmly to the surface said outside adhesive layer adhered firmly to the inner surface of the metal blank having a width larger than that of the lap side seam, and extending in the circumferential direction beyond the turning point of the inside adhesive layer at the seam portion and thus being firmly adhered to the inside of the metal blank.

3 Claims, 15 Drawing Figures METHOD FOR PRODUCING TUBULAR METAL CAN BODY This application is a division of our copending application Ser. No. 153,290, filed June 15, 1971, now U.S. Pat. No. 3,763,895.

This invention relates to a-tubular metal can body in which the marginal portions of a metal blank are joined by a lap side seam through an adhesive layer of an organic polymer, and more specifically, to a tubular metal can body in which the metal is effectively prevented from being dissolved into the contents of the can at the cut end portion of the marginal portion, and also to a method of producing such tubular can body at high speed and with precision.

The term tubular, used in the present specification and claims, is meant to express not only a cylindrical form, but also a cross-sectionally elliptical form or a cross-sectionally polygonal form.

Metal can bodies are generally produced by forming a metal blank such as tin plate into a cylindrical shape, and joining the marginal portions by some means. Metal cans which have been most widely used are those produced by joining the marginal portions of a cylindrical body formed from a metal blank such as tin plate by soldering.

In recent years, the tin-free steel represented by steel plate treated electrolytically chromic acid has become available at low cost as a can making material, and has widely superseded the tin plate. This tin-free steel cannot be soldered, and therefore, a technique has been developed for producing can bodies by bonding the marginal portions of this blank with a thermoplastic adhesive such as nylons.

Various techniques of forming a side seam of the can body using an adhesive have been proposed. One of such techniques is called hook seam technique which involves forming a rectangular thin sheet as a blank of a can body into a tubular shape, bending both marginal portions along the longitudinal direction in the opposite directions by a known method to form hooks, pouring a heated thermoplastic resin on one of these hooks (generally the hook bent outwardly), engaging it with the other hook while it is still in the plastic state at high temperatures, clamping both hooks tightly by bumping, and allowing the thermoplastic resin to cool and solidify, thereby to complete the hook-seam clamping.

In this hook-seam type can body, the cut end of the marginal portion of a metal blank is embedded in the adhesive, and do not directly contact the contents of the can. Hence, it has the advantage that the metal is not dissolved into thecontents of the can. However, the hook portions are readily deformed, and the strength of the joint is limited. Such hook seam type can bodies are unsuitable for use in fields where a high strength of the joint is required, such as pressure cans for including beer or soft drinks.

Another type of joining in the production of a can body comprises melt-adhering a tape-like thermoplastic organic adhesive to the opposite surfaces of both marginal portions of a rectangular thin metal sheet as a blank, bending the metal blank in a cylindrical shape, heating both marginal portions to which said organic adhesive has been adhered, superposing both marginal portions so that the adhesives on the marginal portions stick to each other, and thereafter bonding the superposed portion, followed by cooling. This lap seam type can body has the advantage that the seam portion has sufficient strength and can withstand use as pressure cans. However, since the cut end surface of the metal blank located inside the can body is exposed in its ascut state, corrosion readily develops at the cut end surface. Furthermore, the metal of the blank is readily dissolved into the contents of the can at the cut end surface, and there is a great tendency that the contents will be contaminated or degenerated.

Various methods have been proposed to protect the cut end portion from exposure to the inside of the can body. For facilitating understanding of the difficulty of protection, the structure of the cut end portion will be described in detail below.

in industrial operation, a thin metal plate is generally cut by utilizing a shearing force caused by two cutting blades, and at this time, a sharp shearing burr occurs unavoidably. A number of apparatuses have been known to remove the shearing burr, and among the methods of removing it are, for instance, (1) a method of collapsing the shearing burr and (2) a method of filing away the shearing burr. Method (1) is effective for the shearing burr of thick plate, but only results in the changing of the projecting direction of the burr when used to cut a thin sheet. When method (2) is used, the protective coating on the surface of the sheet is cut away at the same time, and it is difficult to completely remove iron powders formed at the time of filing. The formation of iron powders is undesirable especially when the sheet is used for production of cans for food. Accordingly, known methods of clearing burrs cannot be used, and one is forced to use as blanks those sheets whose cut end portions include both cut end surfaces and shearing burrs.

One conceivable method of preventing the cut end portion of a metal blank from being exposed to the inside of the can body comprises forming a metal blank into a tubular can body by the above-mentioned process, and coating a lacquer on the cut end portion exposed at the side seam, thereby to protect it. This method requires a coating step and a step of drying the coating, and the drying step must be carried out at a temperature below the melting point of the adhesive. Lacquers suitable for use in this method are very much limited. Even with lacquers which dry at a temperature below the melting point of the adhesive, it is difficult to apply the lacquers only to the cut end portion of the side seam, and in order to protect the cut end portion, the coating of the lacquer should be made thick. With increasing thickness of the coating, there is a greater tendency of foaming of the coating when the solvent is dried. In addition, it is difficult to coat burrs having acute angles with a liquid substance, and a can body is difficult to produce in which the cut end portion is completely protected.

In another proposal previously made, a can body is formed by joining both marginal portions of a metal blank with an adhesive, and then a molten thermoplastic resin is poured onto the exposed metal portion inside the seam so as to cover the exposed metal portion. However, at the exposed metal portion of the seam of the can body, there is a stepped shoulder between the inner surface of the metal blank situated outside the seam and the inner surface thereof, situated inside the seam, which shoulder is due to the thickness of the adhesive layer and that of the metal blank. This portion containing the stepped shoulder is difficult to protect by coating it completely with a melt of a thermoplastic resin which has high viscosity and poor wetting properties. In a can structure of this type in which the stepped shoulder portion of the seam cannot be completely and firmly covered and protected, voids occur at the stepped shoulder portion owing to the thickness of the metal sheet and that of the adhesive layer. If an article is stored in such a can, liquid infiltrates into the voids, and corrodes the exposed metal surface with the lapse of time, which in turn results in the dissolving of metal and after a long time, causes it to dissolve in the article stored.

Furthermore, a method of coating the exposed metal portion of the seam of the can body by pouring a lacquer or a molten thermoplastic resin into it cannot be free from such defects as the formation of pin holes at the coated part of the shearing burrs or poor adhesion.

Accordingly, an object of this invention is to provide a tubular can body wherein the cut end portion of a marginal portion of the metal blank located inside the can body, especially the cut end surface and the shearing burr, are .completely covered with a coating of a thermoplastic organic polymer firmly adhered thereto, and consequently, the dissolving of the metal in the contents of the can is markedly inhibited.

Another object of the invention is to provide a method of producing the above-mentioned can body at high speed and without complicated operation.

According to the present invention, there is provided a tubular metal can body wherein the dissolving of metal in the contents of the can is prevented, said can body comprising a metal blank whose longitudinal marginal portions are joined to each other by a lap side seam through adhesive layers of a thermoplastic organic polymer, said both marginal portions of the metal blank each including two opposing surfaces, out end surface and shearing burr projecting from one corner of the cut end portion, said lap side seam being formed by the integral joining of an inside adhesive layer in a continuous tape form of a thickness from to 200 microns which completely covers both the inner and outer surfaces of the marginal portion of the metal blank located inside the can body, cut end surface and shearing burr and adheres firmly to these surfaces, and an outside adhesive layer in a continuous tape form of a thickness of 5 to 100 microns which covers at least the inner surface of the marginal portion of the metal blank located outside the can body and adheres firmly to the surface said outside adhesive layer adhered firmly to the inner surface of the metal blank having a width larger than that of the lap side seam, and extending in the circumferential direction beyond the turning point of the inside adhesive layer at the seam portion and thus being firmly adhered to the inside of the metal blank.

The invention will be described with reference to the drawing in which:

FIG. 1 is a perspective view of a metal blank to be used for the production of the can body of the present invention;

FIG. 2 is a perspective view of another metal blank to be used for the production of the can body of the present invention;

FIG. 3 is a perspective view of the metal blank in an exploded form in which an inside adhesive layer and an outside adhesive layer have been applied toboth marginal portions;

FIG. 4 is a perspective view showing another embodiment of the metal blank for the production of the can body, which is the same as FIG. 3 except the structure of the outside adhesive layer;

FIG. 5 is a perspective view of a tubular can body according to the present invention;

FIG. 6 is an enlarged sectional view showing the section in the circumferential direction of the lap side seam of the tubular can body of the invention;

FIG. 7 is an enlarged sectional view showing the section in the circumferential direction of the lap side seam of the tubular can body according to another embodiment of the invention;

FIG. 8 is a simplified arrangement of an apparatus for applying an adhesive tape which is used in the practice of the method of the present invention;

FIGS. 9-A, 9-B, 9-C and 9-D are views explaining the operation of covering the marginal portion of the metal blank with an adhesive tape;

FIG. 10 is a simplified arrangement of a roll form station for forming metal blank into a cylindrical shape;

FIG. 11 is a simplified arrangement of a heating station for heating the marginal portions of metal blank formed into a cylindrical shape; and

FIG. 12 is a simplified arrangement of a bumping station for joining the superposed portion of both marginal portions of metal blank.

The metal blank to be used in the invention may be any metal blanks used conventionally for the production of can bodies, which include, for example, low carbon steel sheets, low carbon steel sheets whose surfaces have been plated with a metal such as tin, aluminum,

zinc or chromium, low carbon steel sheets whose surfaces have been treated with phosphoric acid or chromic acid electrolyticuly or non-electrolytically, and aluminum sheets. These metal blanks may be coated with a primer in order to protect the inner surface of the can body, render the outer surface easily printable, and to improve the adhesion at the lap side seam. The metal blanks are used for can making in the form of a rectangular sheet or a long strip.

Referring to FIG. 1, a metal blank 1 includes opposing surfaces 3,3 and a cut end surface 4 at marginal portions 2,2, and also a shearing burr 5- projecting from the corner of the cut end portion which burr is unavoidably formed at the time of shearing the metal blank to the desired size and is not permitted to be removed by any known apparatus. As shown in FIG. 2, in a metal blank 1' consisting of a metal sheet I" and primer coatings 6,6 on the surface thereof, cut end surface 4 and shearing burr 5 at both marginal portions 2,2 are exposed without being covered by the primer coatings 6,6.

It is generally difficult to protect the metal from exposure by adhering a polymer coating firmly to the cut end surface 4 and the shearing burr 5. In particular, the shearing burr 5 projects at an acute angle from the surface of the metal, and therefore, pin holes occur in the protecting coating or the adhesion of the coating becomes poor.

According to the present invention, the cut end surface and shearing burr at the marginal portion of metal blank are firmly adhered to a coating of a thermoplastic organic polymer which acts as an adhesive, and the metal is effectively prevented from exposure.

In FIG. 3, an inside adhesive layer 7 in a continuous ribbon form is provided at the marginal portion 2 of metal blank 1 (for the simplicity of explanation, a metal blank coated with a primer will also be indicated by the reference numeral 1 hereinafter), which adhesive layer completely covers the inner and outer surfaces 3,3, cut end surface 4 and shearing burr 5, and adheres firmly to these surfaces.

The inner adhesive layer 7 is a tape of a thermoplastic organic polymer. This tape covers one surface 3' of the marginal portion of the metal blank with a certain width, and the shearing burr 5 projecting from the corner of the cut end portion. It is bent at the corner of the cut end portion so as to cover the cut end surface 4 and also the surface 3 of the marginal portion with a certain width.

It is especially important that the thickness of the inside adhesive layer 7 should be to 200 microns, in order to cover the shearing burrs 5 and prevent leakage from the lap side seam in the can end seaming process. If the thickness of the inside adhesive layer is smaller than 15 microns, there is an increasing tendency towards the formation of pin holes in the adhesive layer at the position of the shearing burr, and it is difficult to protect the shearing burr completely. On the other hand, larger thickness of the inside adhesive layer in excess of 200 microns does not give any appreciable increase in coating effect, and is economically disadvantageous. At the same time, there is an increasing tendency towards leakage from the lap side scam in the can end seaming process.

The inside adhesive layer is formed by the meltadhesion of the above-described thermoplastic organic polymer tape to the marginal portions of metal blank. To this end, the inside adhesive layer consists of a thermoplastic adhesive known per se. Because of relatively low melting point and high bond strength, linear, aliphatic polyamides having a relative viscosity (measured with respect to a solution of 1 g polymer in 100 cc sulfuric acid) of at least 1.5 are preferred as the thermoplastic adhesive.

The relative viscosity is used herein in an ordinary sense, and is defined as the ratio of the flowing time of a solution of polyamide in 98 percent concentrated sulfuric acid with a polyamide concentration of 1 g/ 100 cc of sulfuric acid, to that of 98 percent concentrated sulfuric acid as pure solvent, as measured at C. by an Ostwald viscometer.

The linear aliphatic polyamides useful in the present invention are those having a relative viscosity at 20 C. of at least 1.5. Polyamides having a relative viscosity less than 1.5 are not preferred since they are brittle and are poor in their ability to protect the cut end surface, and shearing burrs by firm adhesion thereto.

Specific examples of the linear aliphatic polyamides used in the invention include:

nylon-6,6 (polyhexamethylene adipamide),

nylon-6,10 (polyhexamethylene sebacamide),

nylon-6,12 (polyhexamethylene dodecamide), nylon-6,13 (polyhexamethylene tridecamide), nylon-10,6 (polydecamethylene adipamide), nylon-10,10 (polydecamethylene sebacamide), nylon-10,12 (polydecamethylene dodecamide), nylon-10,13 (polydecamethylene tridecamide), nylon-12,6 (polydodecamethylene adipamide), nylon-12,10 (polydodecamethylene sebacamide nylon-12,12 (polydodecamethylene dodecamide),

nylon-12,13 (polydodecamethylene tridecamide), nylon-13,6 (polytridecamethylene adipamide), nylon-13,10 (polytridecamethylene sebacamide), nylon-13,12 (polytridecamethylene dodecamide), nylon-13,13 (polytridecamethylene tridecamide nylon-6,9 (polyhexamethylene azelamide), nylon-10,9 (polydecamethylene azelamide), nylon-12,9 (polydodecamethylene azelamide), nylon-13,9 (polytridecamethylene azelamide), nylon-6 (polymer of omega-aminocaproic acid), nylon-7 (polymer of omega-aminoheptanoic acid), nylon-8 (polymer of omega-aminocaprylic acid), nylon-9 (polymer of omega-aminopelargonic acid), nylon-l0 (polymer of omega-aminodecanoic acid), nylon-11 (polymer of omega-aminoundecanoic acid),

nylon-12 (polymer acid),

nylon-l3 acid), lnterpolyamides of the respective monomers, and mixtures of the polyamides described above and the interpolyamides may also be used. Preferred polyamides are those having low hygroscopicity, and include polyhexamethylene sebacamide, polydodecamethylene adipamide, polydodecamethylene sebacamide, polyhexamethylene tridecamide, polytridecamethylene tridecamide, polymer of omega-aminoundecanoic acid, polymer of omega-aminododecanoic acid, polymer of omega-aminotridecanoic acid, and interpolymers consisting predominantly of said polyamides.

These linear polyamides may be used as admixtures, or may contain additives such as plasticizers and heat stabilizers.

For improving the adhesion of the adhesive layer to the surface of metal sheet, it is preferable to use a metal blank having a primer coating of 2 10 micron thick ness as a protective coating. The primer coating used for this purpose may be of any material conventionally used for coating metal sheets for can making, and include, for example, coatings of thermosetting epoxy resins such as combinations of bisepoxy compounds with amines, organic acids, acid anhydrides, phenolic resins, urea resins or polyamide resins. The primer coatings especially preferred in the present invention are the so-called epoxy-phenol resin coatings consisting of resol type phenol resins and epoxy resins. Such epoxy-phenol resins are the precondensation between (A) resol-type mixed phenol resins of monofunctional phenols, namely p-alkylphenols such as p-cresol, and polyfunctional phenols such as phenol, meta-cresol, resorcinol, m-methoxy phenol, m-ethoxy phenol, moctyloxy phenol, p,p'-dihydroxydiphenyldimethylmethane (bisphenol A), the ratio of the monofunctional phenol to polyfunctional phenol being 50:90 to :10, and (B) bis-epoxy compounds consisting of epichlorohydrin and bisphenol A having an epoxy equivalent of 450 to 5,500, the ratio of (A) to (B) being 20:80 to 60:40.

Turning now to FIG. 3, an outside adhesive layer 8 in the form of a continuous ribbon is provided on the other marginal portion 2 of the metal blank 1 so as to cover at least the inner surface 3 of the marginal portion 2' and adhere firmly thereto. Conveniently, the outside adhesive layer, as shown in FIG. 3, may be provided only on the inner surface 3 in a ribbon form of a certain width from the cut end surface 4 of the marof omega-aminododecanoic (polymer of omega-aminotridecanoic ginal portion 2', It may also be provided as shown in FIG. 4 so as to completely cover the opposing surfaces 3,3, cut end surface 4 and shearing burr atthe marginal portion 2' of the metal blank and adhere firmly to these surfaces, just as in the case of the inside adhesive layer 7. Even when the outside adhesive layer 8 is provided only on the inner surface of the marginal portion 2' of the blank, the metal can be effectively prevented from being exposed to the inside of the can body, and there can be obtained sufficient lap joining strength that is endurable in uses as pressure cans. Of course, when the outside adhesive layer is provided on the entire marginal portion 2 as shown in FIG. 4, it is also possible to prevent the metal from being exposed outside the lap side seam of the can body.

The outside adhesive layer is also formed by the hot melt-adhesion of a tape of a thermoplastic organic polymer, and the material same as that of the inside adhesive layer can be conveniently used. It is important that the thickness of the outside adhesive layer should be to 100 microns. By providing an outside adhesive layer of the described thickness on the marginal portion of the metal blank located outside the can body, there can be formed a strong lap joint irrespective of the existence of the shearing burr at the marginal portion, and the occurrence of pin holes in the adhesive layer at the shearing burr or the poor adhesion is not brought about.

When the thickness of the outside adhesive layer is smaller than 5 microns, poor adhesion often occurs at points of the lap side seam at the time of high speed can making, namely at the time of producing a can body by bumping overlapping marginal portions of a blank. On the other hand, if the thickness of the outside adhesive layer is larger than 100 microns, the contents of a can tend to leak out from the juncture portion of the lap side seam and the closure end which have been double seamed.

The formation of a lap side seam by integrating the inside adhesive layer and the outside adhesive layer of the specified thickness brings about the following outstanding advantages.

When an adhesive layer is provided only on one of both marginal portions of a metal blank, the adhesive layer should have a sufficient thickness to compensate for the unevenness of the surface of the metal blank in order to cause both marginal portions to adhere firmly to each other irrespective of the uneveness of the surface of the blank at the time of bumping. On the other hand, if the thickness of the adhesive layer becomes larger, heat transmission of the adhesive layer becomes poor, in which case it is necessary to heat the marginal portions of the metal blank having the adhesive layer to a higher temperature in order to render the surface of the adhesive layer tacky. If the marginal portions of the metal blank are heated to higher temperatures, cooling of the adhesive layer takes a longer time in the formation of a lap side seam by bumping both marginal portions, and hence, the can-making efficiency is reduced.

Since in the present invention the outside adhesive layer and the inside adhesive layer are integrated at the lap side seam, heat transmission to the surface of each adhesive layer can be effected easily, and the cycle of heating and cooling the adhesive layers can be performed within a shorter period of time.

As shown in FIG. 5, the tubular can body of the invention is made of a metal blank 1 formed in a cylindrical shape, and both marginal portions 2,2 in its longitudinal direction are joined to each other by a lap side seam integrally united by an inside adhesive layer to cover the inside marginal portion 2 and an outside adhesive layer 8 to cover at least the inner surface of the outside marginal portion 2'. The inside adhesive layer 7 and the outside adhesive layer 8 are integrated by melting at their interface 9, and there is no visible boundary line at the lap side seam of the can body.

As is shown in FIGS. 5 to 7, it is especially preferred that the outside adhesive layer 8 should be provided with a width larger than that of the lap side seam and extend in the circumferential direction beyond the turning point of the inside adhesive layer at the seam portion. By providing the outside adhesive layer to extend in the circumferential direction beyond the turning point of the inside adhesive layer in the seam and to adhere firmly to the inner surface of the metal blank, abrupt changes in the thickness of the can body in the circumferential direction can be reduced to some extent by the thickness of the outside adhesive layer 10. It is possible therefore to prevent seaming rolls from jumping in the double seam process owing to the presence of the stepped shoulder and forming a part that is weakly seamed, and thus to avoid the cause of leakage. If the outside adhesive layer is retreating from the turning point of the inside adhesive layer, there is formed a small gap surrounded by three surfaces, i.e., the cut end surface of the outside layer, the turning end of the inside adhesive layer, and the inner surface of the edge of the metal blank, and it frequently becomes difficult to fill this gap completely with a sealing material coated on the closure end. This often becomes a cause of leakage. The positive projection of the outside layer beyond the turning point of the inside adhesive layer ensures complete prevention of the formation of such a gap. Accordingly, leakage from the secured portion between the can body and the closure end can be prevented, and the closure end can be secured to the can body easily without failure.

It might be possible to extend the outside adhesive layer in the circumferential direction to the other marginal portion of the metal blank to cover the entire inner surface of the metal can body as if with a primer coating. In such a can body, however, the thickness of the outside adhesive layer overlaps the thickness of the inside adhesive layer, and abrupt changes in the thickness of the can body in the circumferential direction cannot be reduced. Accordingly, the extension in the circumferential direction of the outside adhesive layer should be to a point where it does not contact the inside adhesive layer. Furthermore, for functional and economic reasons, it should not be extended more than necessary, and desirably the extension is to about twice the width of the seam.

The width of the inside adhesive layer may be the same as that of the lap side seam. From the standpoint of easy operation of forming the lap side seam and easy securing of the closure end to the can body by double seaming, the outside part of the inside adhesive layer is preferably provided to extend to some extent in the circumferential direction beyond the cut end portion of the metal blank located outside the can body. There has been provided a method of filling a gap between the stepped shoulder and the closure end by heating this portion at the time of end seaming, and pouring a molten thermoplastic adhesive into the gap. This method may be usable for the end seaming of a can body free from contents, but when the can body contains an article in it, it is difficult to use this method. Especially when the end seaming process must be done with the contents kept cold, such a method cannot be employed. The can body of the present invention is superior in that leakage is prevented by the seam structure.

The preferred dimensions of the individual parts of the lap side seam of the tubular can body of the invention are as follows:

Especially preferred range Thickness of the metal blank (d) 0.12-0.35 mm 0.13-0.32 mm Thickness of the inside adhesive layer (11,) 15-200 a 30-60 p. Thickness of the outside adhesive layer (d -100 p. 30-80 a Adhesion width of the lap joint scam (I) 3.5-8 mm 46.5 mm Extended width of the outside adhesive layer (1,) Extended width of the inside adhesive layer h/(Zd, d) 1-20 4-12 l /(2d d) 1-20 4-12 As shown in FIG. 6, the lap side seam of the tubular can body of the invention may be formed so that the shearing burr 5 at the marginal portion of metal blank located inside the can body faces inwardly, and the shearing burr 5' of metal blank located outside the can body faces outwardly.

In the most preferred embodiment of the invention, however, the shearing burr 5 faces outwardly and the shearing burr 5 inwardly as shown in FIG. 7; in other words, the two projecting shearing burrs 5 and 5' at both marginal portions of the metal blank face each other with the inside and outside adhesive layers 7 and 8 therebetween. By the provision of the shearing burrs 5 and 5 as shown in FIG. 7, the formation of pin holes in the protective coating of the shearing burrs or poor adhesion can be completely prevented.

With the conventional lap seam type can bodies, the arrangement of shearing burrs as shown in FIG. 7 is likely to result in the injury of the surface of metal blank, especially the protective coating when the metal blank is formed into a cylindrical shape and the marginal portions are lapped and joined to each other. According to the present invention, the exposure of the shearing burrs can be effectively prevented without involving the above-mentioned defects because the inside marginal portion of the metal blank is completely protected by a coating of the inside adhesive layer, and the outside adhesive layer is provided on the inner surface of the outside marginal portion.

For facilitating the understanding of the invention in greater detail, the invention will be described by the following embodiment.

EXAMPLE 1 Both surfaces of a large steel sheet having a thickness of 0.2 mm which had been treated electrolytically with chromic acid (steel manufactured by Toyo Kokan Kabushiki Kaisha and sold under the tradename Hi- Top") were coated with an epoxy-phenol lacquer in a thickness of 5 u, and the coating was baked at 210 C. for 10 minutes. The steel sheet so treated was sheared into rectangular blanks each of 210 mm X mm in size. As shown in FIG. 3, an adhesive tape of a lauryl lactam polymer nylon 12 (relative viscosity 2.4) having a thickness of 40 p. and a width of 12 mm was adhered to one surface held at 220 C. of one edge in 125 mm length of this metal sheet and then folded to adhere to the cut end surface and shearing burr and thus protect them. A film of the same adhesive having a thickness of 60 p. and a width of 6 mm was firmly adhered to one surface of the other marginal portion. The blank was then formed into a tubular shape. The adhesive layers were bonded to each other at 220 C., as shown in FIGS. 5 and 7, followed by cooling, to form a can body. The width of the superposed portion was 5 mm.

A bottom end was secured to the can body by the conventional double-seaming process, and an article was filled into the can. A top end was fixed to the can. The can was then stored at 50 C. for 20 days, and the amount of iron that was dissolved in the contents of the can was measured. The results are given in Table 1.

For comparison, an adhesive tape of the same material as described above having a thickness of 60 ,u and a width of 6 mm was adhered to one surface of each marginal portion of the same coated steel sheet as used in the Example above. The marginal portions were bonded at 220 C. so that the adhesive layers were superposed, and then cooled. Thus, there was obtained a lap seam joined can whose cut end surface and shearing burr were not protected by an adhesive. The cut end surface and the burr were then protected by polyvinyl chloride lacquer (can of comparison 1) or Versalon 1 165 (trademark for a polyamide resin having a softening point of 165 C.) (can of comparison 2). The amount of iron of this can which dissolved in the contents is also shown in Table 1. When the cut end surface and shearing burr were protected by the polyvinyl chloride lacquer, the solids content of the lacquer was 20 percent. The lacquer was coated on the cut end portions, and dried at C. for 5 minutes. In the case of Versalon 1165, it was melted at 200 C., and the melt was applied to the cut end portion and then cooled.

TABLE 1 Quantity of dissolved iron (ppm) Can of Can of (.an of invention, comparison comparison Can Contents Example I 1 Beer less than 0.02 0.58 0.28 Cola beverage 0.02 6.4 3.1 4% aqueous sodium less than chloride solution 0.02 1.7 0.9 (rusting) (rusting) 0.5% aqueous citrate buffer solution 0.02 25.1 14.3 4% aqueous acetic acid solution 0.12 50.4 28.4

It is seen from the resultsgiyen in Table 1 that in terms of the amount of dissolved iron, the can of the present invention is superior for use in storing any article to those outside the scope of the present invention.

The can bodies of the invention are useful for various applications. In view of the inhibited dissolution of metal and the freedom from leakage, they are especially useful as cans for foods and drinks. For this purpose, it is preferred to use metal blanks produced by applying a coating of the epoxy-phenol resin on the tinfree steel sheets such as low carbon steel sheet treated with chromic acid. The can bodies having a lap side seam according to the invention find particular utility as pressure cans for including beer or soft drinks.

The can body of the invention can be prepared by any desired technique. In a preferred embodiment, it is produced by the method and apparatus to be described below in detail.

According to the present invention, there is provided a method of producing at high speed and with precision a tubular can body wherein both marginal portions of metal blank are joined to each other by a lap side seam through adhesive layers of a thermoplastic organic polymer, said method comprising:

a. heating both marginal portions of a metal blank by induction heating;

b. feeding a continuous tape of a thermoplastic organic polymer having a thickness of to 100 microns to at least the inner surface of one of said marginal portions to be located outside the can body, along its longitudinal direction, thereby to form an outside adhesive layer melt-adhered to said marginal portion;

c. feeding a continuous tape of a thermoplastic organic polymer having a thickness of 15 to 200 microns to one of the inner surface, outer surface and cut end surface of the other marginal portion to be located inside the can body, along its longitudinal length, causing a part of said tape in its widthwise direction to adhere firmly to one surface of said marginal portion at a temperature at which the surface of said tape contacting the metal blank is melted but the opposite surface of said tape is not substantially melted, and then applying an inwardly bending force to said tape firmly adhered to one surface of said marginal portion thereby to form an inside adhesive layer which firmly adheres to and covers the inner surface, outer surface, cut end surface, and shearing burr;

d. forming the resulting metal blank having the adhesive layers at both marginal portions into a substantially tubular shape;

e. heating the said marginal portions of the metal blank formed substantially in a tubular shape by induction heating at a temperature sufficient to render said adhesive layers tacky, and

f. superposing the inside marginal portion of the metal blank on its outside marginal portion, and pressing them with cooling.

The method of the invention will now be described in detail with reference to FIGS. 8 to 12.

According to the method of the invention, both marginal portions of metal blank are heated in order to provide an inside adhesive layer and an outside adhesive layer thereon. Referring, for instance, to FIG. 8, a metal strip 1 is continuously fed into an induction heating device 11, whereby both marginal portions 2 and 2 are heated to a predetermined temperature. The heating of the marginal portions of metal blank may also be performed by contact with gas flame or heated rollers, or by irradiation of an electric heater. The induction heating is superior in that it can selectively heat the marginal portionsuniformly to a predetermined temperature. The heating temperature for the marginal portions of the metal blank is the melting point of the thermoplastic organic polymer used as adhesives or higher.

An adhesive tape 12 of a thermoplastic resin is fed to one marginal portion 2 heated. By a pair of applicator rollers 13, the tape is firmly adhered to the top surface of the marginal portion 2' of the metal blank, whereby an outside adhesive layer 8 melt-adhered to the marginal portion 2' is formed.

On the other hand, an adhesive tape 12' of a thermoplastic organic polymer is fed to one (bottom surface in the drawings) of the inner surface, outer surface and cut end surface of the other marginal portion 2 heated. Only a part of the tape 12' is firmly adhered to one surface of the marginal portion 2 by a pair of the applicator rollers 13. At this time, the press-contact between the adhesive tape 12' and the surface of the marginal portion 2 of the metal blank should be effected at a temperature at which the surface of the adhesive tape contacting the metal blank is melted but the opposite surface of the tape is not substantially melted. This is especially important for adhering the adhesive layer firmly to the marginal portion of the metal blank at high speed and with precision, and for avoiding the film damage in the subsequent folding step. This condition can be experimentally determined by adjusting the relative positions of the induction heating device 11, the applicator rollers 13, and folding members 14a, 14b and 140 or by controlling the induction heating temperature. Preferably, the applicator rollers are cooled for preventing the breakage of the film. An inwardly bending force is applied to the remainder of the tape firmly adhered to one surface of the marginal portion, thereby to form an inside adhesive layer 7 which firmly adheres to and covers the inner and outer surfaces, cut end surface and shearing burr of said marginal portion. Preferably, the inwardly bending force is successively applied to the adhesive tape projecting from the marginal portion, as shown in FIGS. 8, and 9-A through 9-D.

In order to apply an inwardly bending force to the projecting part of the tape, the projecting part is brought into contact with, for instance, a member having a surface which acts to bend the projecting part of the tape obliquely inwardly in a progressive manner. In FIGS. 8, and 9-B, 9-C and 9-D, such a member consists of a plurality of processing rollers 14a, 14b, 14c mounted at different angles. A first processing roller 14a has an actuating surface inclined at an angle less than to the surface of the metal blank, and causes adhesive tape 12a to adhere firmly to shearing burr 5 of the metal blank (see FIG. 9-B). A second processing roller 14b has an actuating surface substantially at right angles to the surface of the metal blank, and causes adhesive tape 12a to adhere firmly to the cut end surface of the marginal portion of the metal blank (see FIG. 9-C). A third processing roller has an actuating surface inclined at an angle larger than 90 to the surface of the metal blank, and bends adhesive tape 12a to the opposite surface of the marginal portion of the metal blank (see FIG. 9-D). The adhesive tape is then fed through a pair of press rollers 16, and firmly adhered to the surface opposite to the marginal portion of the metal blank (see FIG. 8.). The number of the roltape projecting from the marginal portion of the metal blank from adhering to the marginal portion of the metal blank irregularly, and to perform the precise folding step by keeping the adhesive tape elongated in a widthwise direction, it is preferred, as shown in FIGS. 8 and 9-A, that a member for preventing irregular folding, for example, roller 15, be provided upstream of the tape folding members 14a, 14b and 140. When air bubbles exist between the adhesive layers and the cut end surface, moisture gathers in the voids with the lapse of long time, which not only causes rusting or corrosion of metal, but also deteriorates the size precision of blanks generally required in high speed operation.

Besides a plurality of processing rollers shown in FIG. 8, the folding member may be any, for instance, a slot guide having an actuating surface inclined progressively inwardly or an air jet whose jetting angle is inclined progressively inwardly.

The metal blank in which the adhesive tapes have been provided at the marginal portions is then fed into induction heating device 17 (FIG. 8), and the adhersion of the inner adhesive layer 7 to the inner and outer surfaces, cut end surface and shearing burr of the marginal portion is completely effected.

The adhesive tape used in the invention may conveniently be of linear aliphatic polyamides having a relative viscosity of at least 1.5 as described above, and the sizes of this tape are as stated above with respect to the can body of the invention. Preferably, the linear aliphatic polyamides used for the adhesive tape have a moisture content of not more than 1.5 percent by weight, particularly not more than 0.8 percent by weight. The polyamide tape generally has a high moisture content. For example, nylon 6, has a moisture content of above 2.5 percent by weight. When this tape is melt-adhered to the marginal portions of metal blank under high speed operating conditions, steam bubbles occurring in the tape have no time to escape out of the tape through the melt of the resin, and thus a number of bubbles remain in the tape. Sometimes, the steam bubbles break to cause poor adhesion of the tape to the marginal portions of the metal blank. It also becomes difficult to effect complete coating sufficient to prevent the metal from dissolving in the contents of the can. However, if the moisture content of the tape is adjusted to less than 1.5 percent, especially less than 0.8 percent prior to the melt-adhesion of the tape to the metal blank, for example, by drying with heat under reduced pressure, the adhesion of it to the metal blank and the coating can be completely effected.

As previously stated, it is important that the adhesive tape should be adhered to the heated marginal portion of the metal blank at a temperature at which the surface of the tape contacting the metal blank is melted but the opposite surface of the tape is not substantially melted. For example, when a tape of a polymer as meltextruded is directly applied to the marginal portions of a metal blank, the tape sticks and tends to be deformed readily. It is difficult therefore to maintain the tape in an elongated state while folding it gradually over the edge portions of the metal blank, and to effect the coating of the tape on the metal blank with an accurate width. According to the present invention, the heating only of the marginal portion of metal blank makes it possible to melt-adhere the tape only at the surface in contact with the metal blank, and leave the other part in an unmelted state. The bending processing of the adhesive tape can be readily performed as mentioned above.

If required, the metal blank is then sheared to the desired size, and the can body blank as shown in FIG. 3 or 4 can be obtained. This can body blank is formed into a substantially tubular shape by a method known per se so that both marginal portions become a lap side seam. As shown in FIG. 10, can body blank la is fed to a roll form station 18 of the can body maker, and advanced towards a bending guide 20 by a pair of drive rollers 19. By the cooperative action of these members, the blank can be formed into a substantially tubular shape.

Both marginal portions of the substantially tubularshaped blank are then heated by induction heating to a temperature sufficient for the adhesive layers to become tacky. The heating is performed by induction heat-ing devices 23 and 24, whereby the marginal portion 2 having the inside adhesive layer and the marginal portion 2' having the outside adhesive layer are heated until they become tacky in heating station 22 shown in FIG. 11 which is provided coaxially with horn 21 of roll form station 18. At this time also, it is preferred that the inside and outside adhesive layers melt-adhered to the metal blanks should have a moisture content of not more than 1.5 percent, especially not more than 0.8 percent, by weight.

In the next operation according to the invention, the inside marginal portion of the can body blank is superposed on the outside marginal portion, and both marginal portions are pressed with cooling, thereby to melt-bond the inside and outside adhesive layers inte grally. To this end, as shown in FIG. 12, the can body blank 1b (FIG. 11) in which the adhesive layers are sticky is fed to a bumping station 25 provided coaxially with the heating station 22 (FIG. II). In the bumping station 25, one marginal portion 2 of the can body blank 1c supported by mandrel 26 is first urged against the mandrel by a closing mold 27, and then the other marginal portion 2 is urged against the mandrel by a closing mold 28, whereby the marginal portion 2 is superposed on the other marginal portion 2'. A hammer 29 then moves upwards to press the superposed part of both marginal portions of the can body blank 1c to cause the integration of the inside adhesive layer and the outside adhesive layer. At the lower portion of the mandrel 26, there is provided a lower spline 32 including a cooling passageway 30 through which to pass a cooling medium, and at the upper portion of the hammer 29, there is provided a cooling passageway 31 through which to pass a cooling medium. These passageways are for the purpose of cooling the integrated adhesive layers at the superposed part. Thus, the can body shown in FIG. 5 is obtained.

The can body maker consisting of the roll form station and the bumping station is of the conventional type which has previously been used to produce soldered cans. This type of can body maker can be easily moditied for the purpose of this invention by providing the heating station before the bumping station, with induction heating devices 23 and 24 as shown in FIG. 11 secured to the heating station, and also providing cooling means fitted to the hammer and lower spline of the bumping station. This known can body maker is so designed that when a can body blank is fed, the marginal portions of the blank coated with the inside and outside adhesive layers advance while making frictional contact with the positioning guide. But the inside adhesive layer can sufficiently endure the contact so long as it meets the thickness requirement mentioned above.

The method of the present invention provides the advantage that the joining of the marginal portions of metal blank can be performed within a short time of 30 to 200 milliseconds. This is because the inside and outside adhesive layers are provided at both marginal portions of the metal blank and the adhesive layer is firmly adhered to the marginal portion of the metal blank located inside the can body.

We claim:

l. A method of producing a seam structure of a tubular can body wherein both the longitudinal margins of a metal blank are previously provided with adhesive 4 layers of a thermoplastic organic polymer, the bonding process being optional, characterized by comprising the steps in combination a. moving horizontally a band of a metal blank in a predetermined direction such that one margin of the band makes the outside of the overlapped margins and the other margin makes the inside of them when the band is made tubular,

b. heating both margins,

c. feeding a continuous tape of a thermoplastic organic polymer to the upper surface of the margin corresponding to the outside margin along its longitudinal direction, thereby to form tape-like outside adhesive layer melt adhered to the margin such that the width of the outside adhesive layer is greater than that of the lap side seam and the extended width in the circumferential direction beyond the reversal point of the inside adhesive layer at the seam portion is l to times the sum of thickness of the two inside adhesive layers and that of the metal blank,

d. feeding another continuous tape of a thermoplastic organic polymer to one of the upper and lower surfaces and the edge of the other margin correspond- 16 ing to the inside margin along its longitudinal length and causing part of the tape in its lateral direction to adhere tightly to said one surface of the margin at a temperature at which the surface of the tape engaging the metal blank is melted but the opposite surface of the tape is not substantially melted, then applying an inwardly bending force to the tape thereby to form a ribbon-like inside adhesive layer which tightly adheres to and covers the upper and lower surfaces, edge and shearing burr such that the width of the inside adhesive layer is greater than that of the lap side seam andthe extended width in the circumferential direction beyond the lap side seam is l to 20 times the sum of thickness of the outside adhesive layer and that of the metal blank,

. forming the resulting metal blank having the adhesive layers at both margins into a substantially tubular shape,

f. heating the margins of the metal blank formed substantially in a tubular shape at a temperature sufficient to render the adhesive layers tacky and g. superposing the inside margin of the metal blank on its outside margin and pressing them with cooling.

2. A method of producing a seam structure of a tubular can body as set forth in claim 1, wherein the tape of a thermoplastic organic polymer is of a linear polyamide and controlled so as to contain a moisture of not more than 0.8% by weight when applied to the metal blank. W Y

3. A method of producing a seam structure of a tubular can body as set forth in claim 2, wherein the tape of a thermoplastic organic polymer on the metal blank is of a linear polyamide and controlled so as to contain a moisture of not more than 0.8 percent by weight just 

1. A method of producing a seam structure of a tubular can body wherein both the longitudinal margins of a metal blank are previously provided with adhesive layers of a thermoplastic organic polymer, the bonding process being optional, characterized by comprising the steps in combination a. moving horizontally a band of a metal blank in a predetermined direction such that one margin of the band makes the outside of the overlapped margins and the other margin makes the inside of them when the band is made tubular, b. heating both margins, c. feeding a continuous tape of a thermoplastic organic polymer to the upper surface of the margin corresponding to the outside margin along its longitudinal direction, thereby to form tapelike outside adhesive layer melt adhered to the margin such that the width of the outside adhesive layer is greater than that of the lap side seam and the extended width in the circumferential direction beyond the reversal point of the inside adhesive layer at the seam portion is 1 to 20 times the sum of thickness of the two inside adhesive layers and that of the metal blank, d. feeding another continuous tape of a thermoplastic organic polymer to one of the upper and lower surfaces and the edge of the other margin corresponding to the inside margin along its longitudinal length and causing part of the tape in its lateral direction to adhere tightly to said one surface of the margin at a temperature at which the surface of the tape engaging the metal blank is melted but the opposite surface of the tape is not substantially melted, then applying an inwardly bending force to the tape thereby to form a ribbon-like inside adhesive layer which tightly adheres to and covers the upper and lower surfaces, edge and shearing burr such that the width of the inside adhesive layer is greater than that of the lap side seam and the extended width in the circumferential direction beyond the lap side seam is 1 to 20 times the sum of thickness of the outside adhesive layer and that of the metal blank, e. forming the resulting metal blank having the adhesive layers at both margins into a substantially tubular shape, f. heating the margins of the metal blank formed substantially in a tubular shape at a temperature sufficient to render the adhesive layers tacky and g. superposing the inside margin of the metal blank on its outside margin and pressing them with cooling.
 2. A method of producing a seam structure of a tubular can body as set forth in claim 1, wherein the tape of a thermoplastic organic polymer is of a linear polyamide and controlled so as to contain a moisture of not more than 0.8% by weight when applied to the metal blank.
 3. A method of producing a seam structure of a tubular can body as set forth in claim 2, wherein the tape of a thermoplastic organic polymer on the metal blank is of a linear polyamide and controlled so as to contain a moisture of not more than 0.8 percent by weight just prior to performing the step (f). 